1. Technical Field
The disclosure relates to an optical film. More particularly, the disclosure relates to a reflective polarizer to increase the use efficiency of light source and decrease the chromaticity differences among various viewing angles.
2. Description of Related Art
Among planar display devices, only liquid crystal displays (LCDs) use linearly polarized light to produce bright, dark and gray levels. Generally, the main structure of a LCD includes LCD panel and backlight module. The basic displaying principle is providing light by the backlight module and producing linearly polarized light after the light penetrates a polarizer. Next, various gray levels between bright and dark states are produced by liquid crystal molecules in various twisted states to display images on the LCD panel.
However, the total brightness of LCDs is only 4-6% of the light intensity provided by their light source. The dichroic polarizer in the LCD panel is one of the main causes of the loss of the light intensity, since only one half of the light can penetrate the dichroic polarizer. Therefore, if the light can be transformed into linearly polarized light being capable of penetrating the dichroic polarizer before the light penetrate the dichroic polarizer, the use efficiency of the incident light can be greatly increased to solve the poor brightness problem of LCDs.
To solve the poor brightness problem above, the most usual way is using various optical films in the backlight module to increase the LCDs' brightness. One way is adapting the reflective polarizing technology to recycle the incident light to increase LCDs' brightness. The reflective polarizing technology includes reflective polarizer, such as Dual Brightness Enhancement Film of 3M, for treating linearly polarized light and cholesteric liquid crystal phase reflective polarizer for treating circular polarized light. The reflective polarizer needs high level multi-layer process, hence the price is expensive. The manufacturing method of cholesteric liquid crystal phase reflective polarizer is simpler; hence is the production cost can be effectively reduced.
The choleteric liquid crystal of the cholesteric liquid crystal phase reflective polarizer is used to separate right and left circular polarized light of circular polarized light. One of the right and left circular polarized light can penetrate the cholesteric liquid crystal layer, and the other one is reflected by the cholesteric liquid crystal layer. The reflective plate of a backlight module, which is usually in LCDs, can be used to reverse the reflected circular polarized light to penetrate the cholesteric liquid crystal layer. Therefore, a single circular polarized light with two times intensity is obtained. If a quarter wave film is attached on the outer surface of the cholesteric liquid crystal layer, the passed circular polarized light can be convert into linearly polarized light with two times intensity. In addition, the direction of the polarization plane of the linearly polarized light is the same as the direction of the optic axis of the polarizer to increase the brightness.
However, the cholesteric liquid crystal phase reflective polarizer has optical anisotropy. Therefore, the color of the reflective light changes with the viewing angle changes to generate the problem of chromaticity difference. FIG. 1 is a diagram of the chromaticity difference changes with the viewing angle changes. In FIG. 1, the chromaticity difference is more serious when the horizontal viewing angle is larger.